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Comet 46P/Wirtanen will streak to within seven million miles of Earth next weekend, giving astronomers a rare opportunity to use an array of ground- and space-based observatories to explore a comet’s nucleus in unprecedented detail.

Comet 46P/Wirtanen will be making a close pass of Earth December 16, 2018. It will be at its closest distance to Earth in over four centuries. Credit: NASA

First discovered in 1948 by astronomer Carl Wirtanen at the University of California’s Lick Observatory, the comet’s current orbital period is nearly 5.5 years. And it’s expected to be visible to the naked eye for the next several weeks.

This will allow astronomers to investigate the inner 200 km around the nucleus, a region we cannot resolve for most comets. As leftovers from the era of solar system formation, comets may provide the most primitive material that we can get our hands on in the solar system.

Cometary nuclei are small chunks of ice and dust that are too small to study directly from afar, Dennis Bodewits, an Auburn University astrophysicist, told me. Thus, to discern their composition remotely, we have to rely on the gas and dust that surrounds the comet to study the composition of its nucleus, he says.

To that end, Bodewits and colleagues are using Hubble and Chandra to study the composition of that gas, but also how it is altered by radiation from the Sun.

In fact, Bodewits is leading two observing campaigns with Hubble. The first combines Chandra and Hubble observations at the same time to study the comet’s interaction with the solar wind. Then, in January, Bodewits and colleagues will use Hubble to measure the comet’s spectrum at ultraviolet (UV) wavelengths. Such UV wavelengths are not accessible from Earth.

We will compare these spectra with observations acquired with the Rosetta Alice instrument, says Bodewits. This will enable astronomers to investigate how processes near the nucleus change the comet’s gas.

“Ultraviolet (UV) light is very good at picking molecules apart,” said Bodewits, who says the resulting molecular fragments and charged particle ions are very reactive and can combine to form new molecules.

In fact, Rosetta discovered that the electrons freed by UV light accelerate these processes much more than previously thought. Bodewits’ Hubble observations will investigate such reactions.

Although conventional theory long held that comets largely resembled dirty snowballs, researchers are coming to understand that in fact, comets are mostly rocky.

The European Space Agency’s (ESA) Rosetta mission unexpectedly found a large amount of molecular oxygen around the comet 67P/Churyumov-Gerasimenko. That is, up to 10 percent compared to water, says Bodewits. If the comet Wirtanen spews out icy grains, Bodewits says he and colleagues would be very curious to see if and how these grains release O2 and furthermore how it is stored in comets.

“If we find no evidence of O2, this could indicate that the oxygen content of comets is indeed linked to their formation,” said Bodewits. “It would imply that Wirtanen formed much closer to the Sun than comets Halley and 67P.”

Bodewits says this prompts the question: was the oxygen there when our solar system formed or was it created afterward through some reaction?

While the planetary science community waits for this new data on Wirtanen, Bodewits says the next major step in cometary science is to bring back material from a comet’s surface.

To that end, he says NASA is currently considering the CAESAR mission concept that would potentially launch in 2024 and take a 100-gram sample of the comet 67P to return to labs on Earth.